Many communications systems transmit streams of data by dividing a data stream into successive portions and assembling each portion together with, for example, addressing information and error detection and correction information to produce a data packet (sometimes alternatively known as a cell or frame). Each data packet is then transmitted independently by modulating an electrical or electromagnetic (radio or optical) signal in accordance with the packet's contents and transmitting the signal via the relevant communications medium to a receiver. Functions such as intermediate storage, error detection and retransmission are performed at the packet level. The format of data within a packet is typically defined in terms of successive functional groups of symbols (such as bits in a binary digital system), where the interval between symbols is defined in terms of the frequency of a clock signal which controls the timing of electrical or electromagnetic signal transitions representing the data symbols. Boundaries between successive packets in a symbol stream may be defined in various ways, such as by providing a fixed pattern of symbols to identify the start or end of a packet.
In asynchronous communications systems, that is systems in which no master clock signal is used to co-ordinate the operation of system nodes, receiving nodes must be able to recognise and synchronise with the signal transitions which represent the data symbols, and to identify the boundaries between successive data packets. Such systems particularly include (but are not limited to) wireless systems in which data is transmitted by modulation of electromagnetic signals, such as r.f. or optical signals. Examples include the Digital Enhanced Cordless Telecommunications (DECT) standard defined by the European Telecommunications Standards Institute (ETSI) in EN 300 444 and associated documents, and the Bluetooth short-range wireless standard defined by the Bluetooth Special Interest Group in the Specification of the Bluetooth System, v1.0 B, December 1999 (available via the URL http://www.bluetooth.com).
It is also desirable to ensure that the numbers of each different kind of symbol (for example binary 0's and 1's) in the transmitted signal are approximately equal, to avoid problems of long-term d.c. offset in electrical circuits carrying the signal.
The need for synchronisation, boundary recognition and d.c. compensation is typically met by commencing each packet with blocks of data symbols with predetermined values. These blocks are known in the Bluetooth standard, for example, as preambles and sync words. The Bluetooth preamble has a value of either 1010 or 0101 (binary), chosen to facilitate d.c. compensation in accordance with whether the least significant bit (LSB) of the sync word which immediately follows the preamble is 1 or 0 respectively. The Bluetooth sync word is a 64-bit word derived in a manner which ensures large Hamming distance between different sync words and provides good auto-correlation properties of the sync word to assist the timing synchronisation process. The sync word is followed by a 4-bit trailer which has a value of 1010 or 0101, depending on whether the most significant bit (MSB) of the sync word is 0 or 1 respectively, to provide for extended d.c. compensation.
The Bluetooth wireless signal is produced by applying a binary frequency-shift keyed (FSK) 0.5BT Gaussian-filtered baseband modulation to a carrier signal in the 2.4 GHz ISM band, such that a binary one is represented by a positive frequency deviation and a binary zero by a negative frequency deviation.
In certain circumstances it is necessary to be able to determine precisely the position of the preamble's LSB, also known as the bit p0, within a Bluetooth signal. For example, the RF Test Specification for conformance testing of Bluetooth-enabled devices defines various tests which depend upon knowledge of the position of the bit p0, to identify the start of signal bursts to be measured or the position of payload data bits within the signal.
In known systems with similar signal formats (such as DECT) this may be accomplished in one of two ways:                Match the bit pattern of the sync word. This is feasible for DECT signals, which have only two possible sync word patterns, but infeasible for Bluetooth signals for which the sync word can be any 64-bit word        Manually adjust a trigger delay setting until a visual display of the Bluetooth signal is aligned with a marker indicating the desired position of the p0 bit. This is a difficult and time-consuming process which is prone to error, and therefore especially unsuitable in the context of testing of equipment on a production line where speed and accuracy of testing are of great significance.        
It is an object of this invention to facilitate the precise and rapid detection of a selected portion of a data packet, such as the preamble (including the bit p0) of a Bluetooth data packet.